Unheralded Sum, the Kenyan trials winner, powered past the Russian Olympic champion on the home straight and threw her arms into the air as she crossed the line in a personal best time of one minute 57.38 seconds.
Savinova was second in 1:57.80 and American Brenda Martinez snatched third (1:57.91) from her compatriot and long-time race leader Alysia Johnson Montano who threw herself over the line before dissolving into tears.
Montano, fastest in the semi-finals, pulled away from the start and led by 10 metres at the bell but was clearly tying up as the athletes rounded the home bend and had nothing left to give in the dash for the line.
1. Eunice Jepkoech Sum (Kenya) 1:57.38
2. Mariya Savinova (Russia) 1:57.80
3. Brenda Martinez (U.S.) 1:57.91
4. Alysia Johnson Montano (U.S.) 1:57.95
5. Ekaterina Poistogova (Russia) 1:58.05
6. Ajee Wilson (U.S.) 1:58.21
7. Nataliia Lupu (Ukraine) 1:59.79
8. Lenka Masna (Czech Republic) 2:00.59